Pattern formation :
Pattern formation is a developmental processes by which cells acquired different identies, depending on their relatives special position within the embryo. Pattern formation ensures that tissues and organs developed in correct place and orientation..
The process by which during development cells become organised the embryo is called pattern formation. All embryo of a given species have a similar structure or body plane.
During development each cell must differentiate according to its position in the embryo, so that the correct cell types arise in the correct place. In other words, cells must know where they are in relation to other cell in the embryo. This is achive by giving each cell a positional value in relation to be principle embryonic excess.
Reasonal specifications describe any mechanism that tells a cell when it is relation to other cells in the embryo, So that it can belive in a manner appropriate for its position.
Several model systems indicate that cells may acquire positional values on the basis of their distance from a source of morphogen.
Morphogen Gradient :
A morphogen is a substance that can influent cell fate having different effects at different concentration. In its simplest model, the positional information along an excess can be generated by the synthesis a morphogen. At a source one of the excess and diffusion away from the source that would set up a morphoge gradient.
Cells at different position along the excess would received different concentration of morphogen and this could induce different pattern of gene expression of different concentration threshold. Such concentration depending pattern of gene expression could represent the address or positional identity of the cell.
Compartment & segmentation :
During development the establishment of an excess is followed by segmentation. The excess is divided into repetative series of similar but independent developmental units. Segments occurs in many species, from the ovule segmentation of the body of insects, to the somaics of vertebrate embryo. These segments can be completed consider as developmental compartments in which the clonal expansion of a particular cell line is constrict.